To put it all in one perspective: It is not worth the cost to upgrade.
You see, the 915 chipset cannot support any of the Core 2 (Core architecture) processors at all. This means that the entire 915 chipset platform, let alone your current Pentium 4 processor, is now obsolete. And forget about putting in an i3 processor: It requires an entirely new desktop board (motherboard) and DDR3 memory; in fact, an i3 processor will not even physically fit the socket on your current desktop board, let alone being electrically incompatible with your board's processor socket. (Your current desktop board uses either DDR2-533 memory or DDR1-400 memory, both of which are now relatively expensive to upgrade compared to faster, incompatible memory.) And 4GB of RAM would not help much if you're running a 32-bit desktop version of Windows or if you're running an early version of the LGA775 Pentium 4 processor that does not support x86-64 (and thus is limited to strictly 32-bit); in fact, 32-bit versions of Windows and 32-bit processors can only see 3.29GB of RAM at most (with all the rest reserved for hardware PCI resources).
As a result, in order to upgrade your processor you will also need to completely replace your system board and all of your system's memory. And if you are still using PATA internal hard drives and optical drives, you may want to replace those with SATA drives as well since many of the new desktop boards have no PATA connectors at all. And you will also have to purchase a legitimate 64-bit copy of Windows since the installation of 4GB or more of RAM will require 64-bit operating systems. In other words, you might as well have effectively bought yourself an entirely new system considering all of those upgrades that you have to perform.
Thanks a ton for putting effort and giving such descriptive information. Things are much clearer to me however just had 2 more queries.
Q1) Can I increase my RAM to 3GB? Will that help in my current 32 bit win XP?
Q2) Considering my need of Visual Studio, movies and sometimes gaming, which of the following is least required to satisfy my these personal needs?
a. P4 core + compatible motherboard
b. P4 quad + compatible motherboard
c. i3 + compatible motherboard
A1) You could increase to 3GB. But to maintain full dual-channel memory controller performance you will have to find two 512MB modules. Using one 1GB module instead will force the MCH's "Flex" memory controller mode which maps 2GB to dual-channel and 1GB to single-channel.
A2) The i3 with a compatible desktop board and all-new DDR3 memory modules would be the most sensible choice right now. This is because LGA775 (the socket that your current P4 sits on) is now near the end of its production life. As such, it will see no new noteworthy processors being introduced before it is retired for good (likely in a few months). Plus, the processors that are currently available for LGA775 are simply too expensive for the performance that they deliver compared to the i3/i5/i7 series processors given that LGA775 itself is on the brink of being EOL'd.
And if you choose an i3 (or an i5-6xx series processor), you will need a motherboard based on an Intel H55, H57 or Q57 chipset in order to use the i3 processor's integrated video/graphics capability. If you use that processor on a motherboard based on an Intel P55 chipset, you will need to purchase a separate PCI-Express graphics card. The i5-750 and the i7-8xx cannot use the integrated video capability of the H55, H57 or Q57 chipset; a separate PCI-Express graphics card is required with those processors.